Quantitative Evaluation of Carbon-enriched Stabilised ...

Quantitative Evaluation of Carbon-enriched Stabilised Austenite in Cast Austempered Ductile Iron of Varying

Section Thickness

*Olawale John Oluyemi, Ibitoye Simeon Ademola and Oluwasegun Kunle Michael

Department of Materials Science and Engineering, Obafemi Awolowo University, Ile-Ife. Nigeria.

*Corresponding author: [email protected]

AbstractThe quantity of carbon-enriched stabilised austenite (γHC) in austempered ductile iron (ADI) with various casting thicknesses was evaluated in this study. Ductile iron grade that conforms to ASTM A536 65-45-12 was cast, specimens of section thickness in the range of 5 to 25 mm were machined, isothermally treated to 820°C austenitising temperature, and austempered at austempering temperature (T ) of 300 and 375°C. The resulting phase distributions A

of treated specimens were characterised using X-Ray Diffraction (XRD) method. The patterns of XRD were found to consist of ferrite (α) and austenite (γ) phases. The percentage volume of carbon-enriched stabilised austenite in the ADI decreased as section thickness increased. It was concluded that the carbon-enriched stabilised austenite in ADI depends on the casting section thickness and austempering temperature.

Keywords: Ductile iron; Aaustempered ductile iron; Carbon stabilised austenite; Acicular ferrite; Casting thickness

in the ADI matrix. Austempered ductile iron undergo two-stage

reaction during austempering transformation. During the first stage, austenite breaks down into acicular ferrite (α) and carbon-enriched stabilised austenite (γHC). Ferrite nucleates and grow into austenite during the transformation process. The nucleation of ferrite leads to rejection of carbon into the surrounding austenite and as a result austenite becomes enriched with carbon (Yang and Putatunda, 2004; Swain et al., 2012; Górny et al., 2014). The enrichment of austenite with carbon becomes increases as more and more ferrite nucleates. At this stage the austenite contains carbon content exceeding 1.0% which is thermally stable at room temperature. Hence, the final microstructure consists of a matrix mixture of ferrite and carbon-enriched stabilised austenite, and dis-persed graphite nodules (Putatunda, 2001; Konečná et al., 2011; Yusuf and Yılmaz, 2011; Méndez et al., 2017). Austempered ductile iron has remarkable properties because of this structure.

If the casting is austempered longer than required, then second stage transformation will commence, where the carbon-enriched stabilised austenite will breaks down into ferrite and -carbide. The presence of ε-carbide after second reaction makes the materials to becomes brittles and affects the mechanical properties of ADI, hence, it must be avoided (Amar, 2012; Costaa et al., 2010). The time between the end of the first reaction and the beginning of the second reaction is referred to as the 'process window' (Bahmani and Elliot, 1994; Yescas et al., 2001).

1. INTRODUCTIONAustempered ductile iron has an optimum combination of high strength and ductility, resistance to wear, toughness and fatigue strength (Yang and Putatunda, 2004; Harding, 2005; Keough and Hayrynen, 2006; Artola et al., 2017; Zanardi et al., 2017). As a result of the combination of these favourable properties, it is used for structural applications in machine building, automobile assembly, war implements and construc-tion machineries (Yang and Putatunda, 2004; Keough and Hayrynen, 2006; Zimba, 2003; Tun and Lwin, 2008; Bixler et al., 2010; Justin and Keough, 2010). These pleasant mechanical properties are possible because ADI structure is a mixture of carbon-enriched stabilised austenite which imparts ductility, and acicular ferrite which imparts strength (Ronus, 1984; Kovacs and Keough, 1993; Kovacs, 1994; Erić et al., 2004; Basso and Sikora, 2012). Thus, the structural form of acicular ferrite, percentage volume fraction of carbon-enriched stabilised austenite, and quantity of carbon in the carbon-enriched stabilised austenite are important microstructural features of ADI. These distinguish microstructures determine the ADI mechanical properties, while the austempering variables (austempering temperature and time) determine the structural form (Erić et al., 2006). The matrix mixture of carbon-enriched stabilised austenite and acicular ferrite gives ADI superior mechanical properties (Erić et al., 2004; Basso and Sikora, 2012; Olawale et al., 2017) when compared it with ductile iron that has pearlite or ferrite matrix (Samuel and Viswanathan, 2008). However, the austempering variables determine the proportion of these two phases

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for a period of 1 h for complete austenitisation of the structure. They were then rapidly quenched into a salt bath maintained at temperatures of 300 and 375°C and were held isothermally at these temperatures for 2 hrs. The specimens were finally air cooled to room temperature.

X-ray diffraction (XRD) method was used to characterised the morphology of the microstructure and phase distribution of as cast and heat treated specimens. A monochromatic Kα radiation of cobalt at 100 mA and 40 kV was used for the X-ray diffraction analysis. The angular 2θ was scanned at a speed of 0.25 and 1° per minute in the range of 30 - 110° and 45 - 120° respectively using Rigaku rotating head anode diffractometer. The profiles were thereafter analysed using a PANalytical X'Pert Pro powder diffractometer in θ-θ configuration with an X'Celerator detector and variable divergence and fixed receiving slits with Fe filtered Co-Kα radiation (λ=1.789Å). Thereafter X'Pert Highscore Plus Software was used for the identification of phases present in the specimens. The integrated intensities and peak positions of (311), (220) and (111) planes of austenite and (211) and (110) planes of BCC ferrite were also obtained from the profile.

The fraction of the carbon-enriched stabilised austenite was evaluated using X-ray diffraction (Xγ)technique (Miller, 1978) according to Equations (1) - (3).

Xγ = 1.4Iγ/(Iα + 1.4Iγ) ……......(1)where

220 211 Iγ = (Iγ + Iγ )/2 ……........….(2)and

211 Iα = Iα ………………............(3)

where is the reflected intensity of the hkl of the hklIγ,α

respective phase. Also, lattice parameters of the austenite ( ) were ɑγ

determined by X-ray measurements. The position of the diffraction peaks was determined and converted to diffracted angles using standard techniques (Cullity, 1978). From the values of lattice parameters, the carbon-enriched stabilised austenite (C ) was γ

computed according to Equation (4) (Ronus and Rundman, 1987).

Cγ = (aγ – 0.3548)/0.00441 …………….(4) At the austenitising temperature the carbon content in the parent austenite (Co) is fixed and was obtained by Equation (5) (Voigt and Loper, 1984):

Co= Tγ/420 – 0.17(%Si) – 0.95…………….(5) where is the austenitising temperature in °C. This Tγ

carbon (C ) was distributed between the ferrite and o

austenite. Therefore,

Co = Xα Cα. + Xγ Cγ……………………(6)

where is the fraction of acicular ferrite in volume, Xα Cα

is the carbon content of acicular ferrite, is the Xγ

The microstructural feature of ADI can be optimised between this window. A short austempering time will lead to formation of martensite because austenite will be inadequately enriched and subsequently lower the carbon content in stabilised austenite. If the austempering time is beyond the time for commence-ment of second stage there will be formation carbide precipitate and again, a reduction in the carbon content of carbon-enriched stabilised austenite. The austempering period between process window will lead to the formation of ausferrite matrix. This structure is the mixture of carbon-enriched austenite (γHC) and acicular ferrite (α). The excellent mechanical properties attrib-uted to ADI is as a results this unique matrix (Bahmani and Elliot, 1994; Yescas et al., 2001).

Several investigations have reported the effects of austempering temperature and time on the carbon-enriched stabilised austenite (Grny et al., 2014; Bayati and Elliot, 1997; Rao and Putatunda, 2003; Vikas et al., 2008; Vaško, 2009; Amran et al., 2010; Vaško, 2011; Erfanian-Naziftoosi, 2012; Atanda et al., 2014). However, the focus of this study is the effect of casting section thickness on the quantity of carbon-enriched stabilised austenite which to the best of our knowledge has not been investigated.

2. MATERIALS AND METHODThe ASTM A536 65-45-12 (1998) grade of ductile iron used in this work was cast using facilities available in Nigerian Foundries Limited, Sango-Ota, Ogun State, Nigeria. The charge makes up consisting of steel scrap (65%), recycle ductile iron (20%) and pig iron (15%) were melted inside 500 kg medium frequency (3000 Hz) induction furnace. Deficiencies in the chemical composition was adjusted by adding petroleum coke and 75 wt.% ferrosilicon. To ensure homogenisation the melt was superheated to 1550°C and thereafter spheroidised at 1450°C. Tundish ladle process was used to perform spheroidisation treatment using a Fe-Si-5.5%Mg alloy. This was followed by post inoculation with 0.6 percent weight granule of 75 wt.% ferrosilicon, after which the melt was poured at 1440°C into green sand molds with a Y-block shape in accordance with ASTM A897/897M-16 standard (2016). The chemical composition of the casting was analysed by photospectrometry. The final chemistry was presented in Table 1.

From the leg part of the Y-blocks, flat specimens of 25 by 80 mm of 5, 10, 15, 20 and 25 mm thickness were machined and from which ADI were produced by an isothermal heat treatment. The heat treatment involved two steps that consisted of austenitising and austempering. The specimens were austeinitised at a temperature of 820°C and were held at the temperature



fraction of carbon-enriched stabilised austenite in volume, and is the carbon content of carbon-Cγ

enriched stabilised austenite. Since carbon dissolved by ferrite is very little ( ≈ 0.002), the carbon content Cα

of parent austenite was estimated according to Equation (7).

Co = Xγ Cγ ……………………………(7)

The beginning of martensite transformation, M , was s

estimated using chemical composition of the material, empirical equations from literature, and isopleths through Fe-C-2.5 wt.%Si phase diagram (Ronus and Rundman, 1987). The temperature in °C was Msestimated as a function of weight percent of the carbon content (C ) present in parent austenite according to 0

Equation (8).

= 400 - 260C ...................................(8) 0MsDue to the presence of manganese, nickel,

chromium, molybdenum and silicon in the chemical composition, the value is somewhat lower. Hence, Ms

different expressions based on the alloying element, were used to estimate the temperature. The Msexpression due to Lui et al. (2001) and Silva et al. (2014) was considered. In this expression the coefficient due to manganese, nickel, chromium, molybdenum and silicon contents are 30.4, 17.7, 12.1, 7.5 and 7.5 respectively. Hence, the estimate of Mswas made according to Equation (9):

= 400 - 260C - 30.4Mn - 17.7Ni - 12.1Cr - 0Ms7.5Mo - 7.5Si ..................(9)

3. RESULTS AND DISCUSSIONThe X-ray diffraction patterns of the specimens for different section thickness are as presented in Figures 1 and 2. The peaks of diffraction existed in 2 ranges from 50.2 - 109.5° which were identified as γ(111), α(110), γ(002), α(200), γ(220), α(211) and γ(311). These peaks correspond to Bragg angles of 50.2°, 52.3°, 58.4°, 77.1°, 88.5°, 99.7° and 109.5° respectively.

The patterns predominantly consisted of peak values of austenite (γ) and ferrite (α) phases which are located near to each other. This suggests the formation of a matrix comprised of a mixture of carbon-enriched stabilised austenite and acicular ferrite often referred to as ausferrite.

The volume fraction of carbon-enriched stabilised austenite (Xγ) and acicular ferrite (Xα) were determined using Equations (1) to (3) and the results obtained are shown in Table 2. The carbon content of the stabilised austenite (Cγ) was estimated according to Equation (4) and the result obtained is shown in Table 3. The content of carbon present in the parent austenite (Co) at the

austenitising temperature was estimated to be 0.571 wt.% (Equation 5). The corresponding temperature of the beginning of martensite transformation, Ms, was estimated to be 219°C using Equation (9). The total carbon in the carbon-enriched stabilised austenite (XγC ) was estimated according to Equation (7) and γ

result is shown in Table 4. The summary of the X-ray diffraction analysis for all the investigated ADI specimens are shown in Table 5.

The volume fraction variation of carbon-enriched stabilised austenite and acicular ferrite with austempering temperatures and casting section thicknesses are shown in Figure 3(a) and (b). From the results the volume fraction of carbon-enriched stabilised austenite decreases steadily with increasing section thickness while acicular ferrite increase with increasing section thickness. Figure 4 shows the effects of austempering temperature on the carbon content of austenite while a change in total carbon in carbon-enriched stabilised austenite is presented in Figure 5.

From Figure 3(a), the austenite content was observed to be higher at high austempering tempera-ture. The trends of volume fraction variation of carbon-enriched stabilised austenite with section thickness were similar in the two investigated austempering temperatures (300 and 375°C). However, at TA of 375°C the volume fraction of carbon-enriched stabi-lised austenite was relatively higher by 18% when compared with TA of 300°C. Ferrite nucleates out of austenite during the austenitising process, dissolves very little carbon (0.002%), and the dissolves carbon diffuse out into the surrounding austenite in order for the ferrite to grow (Tanmoy and Goutam, 2018). Since nucleation depends on the degree of supercooling, the lower the austempering temperature the higher the degree of supercooling and the more theferrite that will be nucleated (Putatunda, 2001; Yang and Putantuda, 2004). However, at the high degree of supercooling diffusion rate of carbon becomes lower and the rate of growth of ferrite slowed down. This is the reasonwhile the quantity of ferrite is more at of 300°C (Figure 3b). At TA of 375°C the nucleation of ferrite was less because of the lower degree of supercooling, diffusion rate of carbon becomes higher, and the rate of grow of ferrite becomes faster. This is the reason the quantity of ferrite is less at TA of 375°C (Figure 3b). Thus, at TA of 300°C the volume fraction of ferrite (Xα) was more than volume fraction of carbon-enriched stabilised austenite (Xγ) while at TA of 375°C Xγ was greater than Xα.

Elements %C %Si %Mn %P %S %Mg

Standard Value 3.5 – 4.0 2.2 – 2.9 0.3 – 0.6 0.05 max 0.25 max 0.02 – 0.06

Measured Value 3.61 2.54 0.40 0.017 0.023 0.05

Table 1: The results of chemical composition of ductile iron produced compared with standard ASTM A536 65-45-12 grade



Figure 1: X-ray diffraction pattern of (a) 5, (b) 10, (c) 15, (d) 20 and (e) 25 mm section thickness austempered at 300°C

Table 2: Percentage volume fraction of carbon-enriched stabilised austenite



Table 3: Carbon content of carbon-enriched stabilised austenite

Table 4: Total carbon of carbon-enriched stabilised austenite

Table 5: Results of x-ray diffraction analysis of investigated ADI



Figure 2: X-ray diffraction pattern of (a) 5, (b) 10, (c) 15, (d) 20 and (e) 25 mm section thickness austempered at 375°C

of 375°C produced higher austenitic carbon content (XγCγ) than the one at 300°C. At a high TA, the carbon content of the austenite increases because diffusion rate of carbon is high at this temperature (Putantuda, 2001). Since diffusion depend on temperature, diffusion rate of carbon was high at TA of 375°C which resulted in carbon partitioning, and consequently higher XγCγ. The value of XγCγ is important because it can be taken as the total carbon in the carbon-enriched stabilised austenite. From Figure 5, the value of austenitic carbon content (XγCγ) decreased as the section thickness increased for each of the investigated austempering temperature, however, at TA of 375°C

At austenitising temperature of 820°C the Ms tempera-ture as estimated by Equation (9) was 219°C. This temperature was lower than the austempering tempera-tures considered in this study and hence there was no room for austenite to transform to martensite. Due to the high temperature difference between TA of 375°C and Ms, large amount of austenite was enriched with carbon while cooling to room temperature. This explained the reason why the volume fraction of X was relatively γ

higher by about 18% at TA of 375°C than T of 300°C.A

As evident from Figure 4 the carbon content of the stabilised austenite (Cγ) at TA of 375°C was more than that at TA of 300°C. The austempering carried out at TA



Figure 4: Influence of section thickness on the carbon content of carbon-enriched stabilised austenite (C ) at γ

different austempering temperature

Figure 5: Influence of section thickness on the total carbon in the carbon-enriched austenite (X C ) at different γ γ

austempering temperature

Figure 3: Influence of section thickness on volume fraction of (a) carbon-enriched stabilised austenite and (b) acicular ferrite at different austempering temperature

this value was more compared to that of TA of 300°C for all the section thickness investigated. Since both carbon-enriched stabilised austenite content and its carbon content were higher at TA of 375°C for all the section thickness, XγCγ was accordingly higher. However, XγCγ decreased as the section thickness increased. At T of 375°C, Cγ and XγCγ were higher A

than that of 300°C by about 6% and 25% respectively. It was observed in this study that T of 375°C A

resulted in higher percentage volume of carbon-enriched stabilised austenite (X ), carbon content in γ

austenite (C ) and total carbon content (X C ) for all γ γ γ

ranges of section thickness than those obtained at T of A

300°C. On the other hand, percentage volume of acicular ferrite (X ) is higher at T of 300°C than those α A

obtained at T of 375°C. However, the amount of A

carbon-enriched stabilised austenite decreased with increase in section thickness while the amount of acicular ferrite increased.

4. CONCLUSIONIt can be concluded from this study that the fraction of volume of carbon-enriched stabilised austenite and acicular ferrite in ADI depends on the section thickness of casting and austempering temperature. Volume fraction of stabilised austenite decreases with increase in section thickness but increases with austempering temperature. Also, total carbon content and carbon of carbon-enriched stabilised austenite decreases with increase in section thickness but increases with austempering temperature. However, the volume


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